The present invention relates to room temperature vulcanizable silicone compositions. More particularly, the present invention relates to one component room temperature vulcanizable silicone compositions having improved shelf stability, improved primerless adhesion, improved thick section cure, and low toxicity characteristics.
Polyalkoxy silanes functionalized with ether, epoxy, isocyanato, cyano, acryloxy, or acyloxy are important silane coupling agents for room temperature vulcanizable (RTV) silicone compositions. These compounds impart excellent primerless adhesion to a wide variety of substrates and do not promote sealant yellowing or modulus build. Thus, their use represents a significant improvement over that of aminofunctional alkoxy silanes which do promote sealant yellowing and modulus build.
The use of adhesion-promoting polyalkoxysilanes functionalized with amino, ether, epoxy, isocyanato, cyano, acryloxy, or acyloxy in alkoxy-terminated polydiorganosiloxane RTV compositions is disclosed, for example, in U.S. Pat. Nos. 4,483,973 and 4,528,353 (both to Lucas et al.). The RTV compositions disclosed in these references contain a dialkyltin dicarboxylate as the condensation cure catalyst.
To provide an RTV composition with optimum palmerless adhesion characteristics, an adhesion-promoting polyalkoxy silane functionalized with one of the groups described above must be present in the composition at a level of at least 0.6 parts per 100 parts of the polyalkoxy-terminated polydiorganosiloxane and, preferably, at a level in the range of from about 1.0 to about 1.6 parts. It has been found, however, that in the presence of a dialkyltin dicarboxylate condensation catalyst, an RTV composition containing the adhesion-promoting polyalkoxysilane at levels of 0.8-1.6 parts per 100 parts of the polyalkoxy-terminated polydiorganosiloxane will have poor shelf-stability, even in the presence of high levels of a disilazane hydroxy scavenger. The term "shelf stability" as used herein refers to the quality of cure after aging of the uncured RTV composition for 24 hours at 100.degree. C. The ASTM sheet physical properties of a "shelf stable" RTV sealant composition are essentially unchanged after storage for 24 hours at 100.degree. C. relative to a room temperature stored sealant.
Dialkyltin dicarboxylates, in the presence of free hydroxy species (e.g., ring-opened epoxysilanes), will cause the polyalkoxy-terminated polymer to degrade. As a result, the RTV composition either will not cure or will cure poorly. In order for a dialkyltin dicarboxylate-catalyzed RTV silicone composition containing an epoxy-functional polyalkoxysilane to be shelf-stable, the epoxy-functional polyalkoxysilane must be present at less than 0.45 parts per 100 parts of the polyalkoxy-terminated polydiorganosiloxane. However, this amount is insufficient to achieve optimum primerless adhesion.
It is desirable, therefore, to provide an RTV silicone composition having both prolonged shelf life and excellent primerless adhesion.
The RTV silicone compositions disclosed in the patents to Lucas et al. (Lucas), cited above, contain greater than 2% by weight of silazane hydroxy scavengers. Consequently, in addition to limitations regarding shelf-life and primerless adhesion, the Lucas RTV compositions have inferior deep section cure compared to RTV compositions which do not contain a silazane hydroxy scavenger. The inferior deep section cure of the Lucas compositions is caused by a reduction in polymer crosslink sites resulting from hydrolysis interactions between polymer endgroups and the internal (CH.sub.3).sub.3 SiO.sub.1/2 group-producing silazane scavengers.
It is therefore further desirable to provide an RTV composition having improved deep section cure.
U.S. Pat. No. 4,517,337 to Lockhart et al. (Lockhart) and U.S. Pat. No. 4,863,992 to Wengrovius et al. (Wengrovius) disclose alkoxy-terminated polydiorganosiloxane RTV silicone compositions containing dialkyltin-bis-diketonate as the cure catalyst. The Wengrovius patent further teaches the use of high levels of an adhesion promoter, which include amino-functional polyalkoxysilanes. The Lockhart and Wengrovius compositions combine superior shelf stability with excellent deep section cure without the use of a silazane scavenger. However, these compositions require a stabilizing amount of excess beta-diketone to prevent methanol/chelate ligand exchange on the tin which would lead to the formation of methoxy-tin equilibration catalysts. For reasons of cost and availability, the beta-diketone of particular commercial interest is 2,4-pentanedione, with the corresponding cure catalyst being dibutyltin-bis-acetylacetonate. However, the Environmental Protection Agency has recently issued a Significant New Use Rule (SNUR) which labels 2,4-pentanedione a potential neurotoxin and mutagen which is developmentally toxic by inhalation. The SNUR essentially prohibits the use of 2,4-pentanedione in any new consumer product, thus greatly restricting the marketability of RTV compositions containing it. Although other, relatively nontoxic beta-diketones, e.g., 2,4-hexanedione, can be used in place of 2,4-pentanedione, such beta-diketones are not commercially available.
Therefore, it is desirable to provide a more readily available substitute compound for 2,4-pentanedione which prevents tin-methoxy species formation and imparts shelf stability to the RTV silicone composition.
It has been found that amino-functional polyalkoxysilanes react with dialkyltin-bis-diketonate catalysts to form free 2,4-pentanedione which, along with its toxicity as discussed above, also has the disadvantage of reacting with iron contaminants in the uncured elastomer composition to produce a yellow color body. In addition, the amino-functional polyalkoxysilanes also promote post-cure instability resulting in modulus build in the cured elastomer composition.
It is further desirable to provide an RTV composition which does not use an amino-functional polyalkoxysilane as an adhesion promoter.